An electron multiplier typically includes an ion impact plate as the first element of the device. This ion impact plate is an integral component of most ion detectors and has the function of converting the input ions, to be detected, into electrons or secondary ions. The emission of low-energy secondary electrons or secondary ions from the impact plate, is the desired response to the plate being struck by the input ions, and forms the principal signal to be amplified by the detector. These secondary electrons or secondary ions are referred to as signal particles.
A specialised form of ion impact plate, referred to as a high energy dynode (HED), has come into common use in commercial mass spectrometers during the past decade. An HED, is an ion impact plate that is maintained at a high electrical potential (typically between 5 kV and 15 kV). Electrons or ions, generated by ions impacting the HED, are focussed onto the 1st dynode (or input area) of any electron multiplying device.
The HED provides two major functions, which have led to its wide use in commercial mass spectrometers:
A. Because of the high potential maintained on the HED, ions acquire considerable energy when approaching its surface. Secondary electron yield and secondary ion yield (defined as the average number of electrons or ions emitted as a result of an ion impact) increases with increased ion impact energy. As a result ions striking an HED generate more signal particles than would otherwise be possible, and, therefore, the HED increases the ion detection sensitivity of the associated electron multiplier and of the instrument using the electron multiplier. This is particularly useful for high mass ions because of the inherent property of most materials to give lower secondary electron yields and secondary ion yields for higher mass ions.
B. When detecting positive ions a negative high voltage is applied to the HED which attracts the incoming ions and repels the secondary electrons generated as a result of the ion impact. Appropriately shaping the HED and/or surrounding electrodes will ensure that most of the electrons are focussed into the associated electron multiplier. When detecting negative ions a positive high voltage is applied to the HED, which attracts the incoming ions and focuses positive secondary ions generated as a result of the ion impact. The same electron/ion optics used to direct the secondary electrons will be effective for the positive secondary ions. The process of conversion from negative to positive ions by the HED is widely used as the critical step in negative ion detection and is an important function provided by the HED. Other methods are employed for negative ion detection but are not as widely used.
One drawback of the HED is its propensity to generate noise or spurious signals (spontaneous output current which is unrelated to input ions), particularly in the presence of a large partial pressure of helium as is common in gas chromatography mass spectrometry (GCMS). A major mechanism causing this noise is the ionization of meta-stable particles or neutral particles in the region of the HED. One of the primary objects of this invention, at least in one or more aspects or embodiments, is to minimise or eliminate this noise.
A further object of this invention, at least in one or more aspects or embodiments, is to increase the sensitivity of an HED and thus increase its usefulness when used in both positive ion detection mode and negative ion detection mode.